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SubjectRe: [RFC PATCH] mm/page_reporting: Adjust threshold according to MAX_ORDER
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On 6/2/21 10:03 AM, Andrew Morton wrote:
> On Tue, 1 Jun 2021 11:33:19 +0800 Gavin Shan <gshan@redhat.com> wrote:
>
>> The PAGE_REPORTING_MIN_ORDER is equal to @pageblock_order, taken as
>> minimal order (threshold) to trigger page reporting. The page reporting
>> is never triggered with the following configurations and settings on
>> aarch64. In the particular scenario, the page reporting won't be triggered
>> until the largest (2 ^ (MAX_ORDER-1)) free area is achieved from the
>> page freeing. The condition is very hard, or even impossible to be met.
>>
>> CONFIG_ARM64_PAGE_SHIFT: 16
>> CONFIG_HUGETLB_PAGE: Y
>> CONFIG_HUGETLB_PAGE_SIZE_VARIABLE: N
>> pageblock_order: 13
>> CONFIG_FORCE_MAX_ZONEORDER: 14
>> MAX_ORDER: 14
>>
>> The issue can be reproduced in VM, running kernel with above configurations
>> and settings. The 'memhog' is used inside the VM to access 512MB anonymous
>> area. The QEMU's RSS doesn't drop accordingly after 'memhog' exits.
>>
>> /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64 \
>> -accel kvm -machine virt,gic-version=host \
>> -cpu host -smp 8,sockets=2,cores=4,threads=1 -m 4096M,maxmem=64G \
>> -object memory-backend-ram,id=mem0,size=2048M \
>> -object memory-backend-ram,id=mem1,size=2048M \
>> -numa node,nodeid=0,cpus=0-3,memdev=mem0 \
>> -numa node,nodeid=1,cpus=4-7,memdev=mem1 \
>> : \
>> -device virtio-balloon-pci,id=balloon0,free-page-reporting=yes
>>
>> This tries to fix the issue by adjusting the threshold to the smaller value
>> of @pageblock_order and (MAX_ORDER/2). With this applied, the QEMU's RSS
>> drops after 'memhog' exits.
>>
>
> Sounds promising, but...
>
>> --- a/mm/page_reporting.h
>> +++ b/mm/page_reporting.h
>> @@ -10,9 +10,10 @@
>> #include <linux/pgtable.h>
>> #include <linux/scatterlist.h>
>>
>> -#define PAGE_REPORTING_MIN_ORDER pageblock_order
>> -
>> #ifdef CONFIG_PAGE_REPORTING
>> +#define PAGE_REPORTING_MIN_ORDER \
>> + min_t(unsigned int, pageblock_order, (MAX_ORDER / 2))
>> +
>> DECLARE_STATIC_KEY_FALSE(page_reporting_enabled);
>> void __page_reporting_notify(void);
>
> Could you please redo this as a regular old static function in
> page_reporting.c? Bonus points for commenting its design ;)
>

Thanks for your time to review, Andrew. Sorry for the delay and late
reply. Yes, I will do if needed after the discussion with David is
finalized.

Thanks,
Gavin

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